SUMMARY: In AM1, conversion of 3-hydroxybutyrate to acetyl-CoA is mediated by an inducible 3-hydroxybutyrate dehydrogenase, an acetoacetate: succinate coenzyme A transferase (specific for succinyl-CoA) and an inducible β-ketothiolase. Ethanol is oxidized to acetate by the same enzymes as are involved in methanol oxidation to formate. An inducible acetyl-CoA synthetase has been partially purified and characterized; it is essential for growth only on ethanol, malonate and acetate plus glyoxylate, as shown by the growth characteristics of a mutant (ICT54) lacking this enzyme. Free acetate is not involved in the assimilation of acetyl-CoA, and hydroxypyruvate reductase is not involved in the oxidation of acetyl-CoA to glyoxylate during growth on 3-hydroxybutyrate. A mutant (ICT51), lacking malate synthase activity, has been isolated and its characteristics indicate that this activity is normally essential for growth of AM1 on ethanol, malonate and 3-hydroxybutyrate, but not for growth on other substrates such as pyruvate, succinate and C compounds. The growth properties of a revertant (ICT51R) and of a mutant lacking malyl-CoA lyase (PCT57) indicate that an alternative route must exist for assimilation of compounds metabolized exclusively by way of acetyl-CoA.


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